Synchronous generator having auxiliary power windings and variable frequency power source and method for use

ABSTRACT

A synchronous generator is disclosed having main power windings and auxiliary power windings, where the auxiliary power winding is coupled to a variable frequency drive system. The variable frequency drive causes the generator to function as a motor and turn a drive shaft to start a gas turbine. Switching circuits are used to connect and disconnect the auxiliary windings of the generator with the variable frequency power supply.

BACKGROUND OF THE INVENTION

[0001] The invention generally relates to the field of synchronous powergenerators, such as those used in combination with gas turbines.Specifically, the invention relates to synchronous generators havingboth a power and auxiliary windings.

[0002] Synchronous power generators are commonly used by power utilitiesto produce electrical energy. Generators generally have a magnetic rotorthat is surrounded by a stationary stator having conductive windings.Rotating magnetic field from the spinning rotor creates electric currentin the armature windings in a stationary stator that surrounds therotor. The current from these windings is output as electrical powerfrom the generator. The stator generally has two or three armaturewindings, each of which have an induced current. These currents aresynchronous, but out-of-phase with each other. The generator producestwo- or three-phase alternating current as electrical power usable byelectric power utilitliy companies.

[0003] Synchronous power generators are often driven by gas turbines.Gas turbines have a rotating drive shaft that is coupled to the driveshaft and rotor of the generator. When running, the gas turbine turnsthe drive shaft and rotor causes the power generator to produceelectricity. In these gas turbine and generator power units, thegenerator is commonly adapted to alternatively function as a startingmotor for the gas turbine. To start the gas turbine, the generator maybe temporarily operated as a motor that is powered from an auxilaryelectrical power source. Once the generator/motor accelerates therotational speed of the drive shaft sufficiently to start the gasturbine, the gas turbine is started. Once started, the gas turbinebegins to output power to the driving the drive shaft and the generator,and the motor is switched back to operate as a generator.

[0004] A variable frequency power supply that drives a generator as amotor to start a gas tubine is referred to as a “static start” drive.The static start variable frequency power supply applies current to thestator windings of the generator. The magnetic fields created by thecurrent in the stator windings cause the generator rotor to turn which,in turn, powers the drive shaft. The power supply gradually increasesthe frequency of the current applied to the stator to increase therotational speed of the rotor. As the rotational speed of the rotor anddrive shaft increases, the turbine is accelerated to its rated startingspeed, and the turbine becomes self-sustaining and generates outputpower to drive the generator.

[0005] The General Electric Company has previously sold and marketed gasturbine and generator power units that have “static start” capabilities.FIG. 1 illustrates a conventional three-phase synchronous generator 10that is coupled to a static start drive 12 which provides variablefrequency power to drive the generator 10 as a motor in order to start agas turbine. The static start drive 12 is switchably coupled to thethree-phase output lines 14 of the armature of the generator. The outputof the generator is normally connected to a closely balanced powertransmission system 16. A disconnect breaker or other switch 18 connectsthe static start drive 12 to the output line 14 of the generator. Whenthe breaker 18 is closed, the static start drive 12 applies power todrive the generator as a motor and start the gas turbine.

[0006] Power to drive the static start drive 12 is provided by anauxiliary power bus 20. The static start drive provides a variablefrequency power to drive the generator as a motor during the gas turbinestart-up mode. Once the gas turbine is running and self-sufficient, adisconnect breaker or switch 18 disconnects the static start drive fromthe output power lines 14 of the generator. The generator ceases being amotor and instead becomes a generator driven by the gas turbine.Electrical power provided by the generator can be applied to the balanceof the power system which requires electrical power from the generator.

[0007] The static start drive is typically formed of non-moving (hencethe term static) solid-state devices such as a load-commutated inverter(LCI) or pulse-width modulated (PWM) drive formed from solid-staterectifiers, diodes and other such devices. The LCI or PWM may be used toprovide a variable frequency power supply from the constant frequencypower supply (typically 50 Hertz (Hz) or 60 Hz) provided from thestation auxiliary power bus 20. The excitation supply 22 is also poweredby the auxiliary power bus 20 and is coupled to a field winding 24 ofthe generator.

[0008] The static start drive system generally uses a variable frequencypower supply having sufficient capacity to be compatible with thegenerator armature winding terminals. For existing gas turbine generatorsystems, the armature voltage is typically in the range of 10 kV to 20kV. The voltage of the static start drive is typically in the range of2.3 kV to 7 kV. This range is sufficiently close to the normal operatingrange of the generator armature voltage for the static start drive to beapplied directly to the main power windings of the generator. Moreover,for a properly-designed static start system, the voltage and currentspecifications of the static start drive matches the electricalcharacteristics of the generator to provide the required acceleratingtorque to the generator rotor. This matching of the static start drivesystem to the armature voltage of the generator is possible forgenerators having normal operating ranges of 10 kV to 20 kV.

[0009]FIG. 2 shows a conventional generator having an auxiliary powerwinding used for exciting the generator when the generator is operatingin generator mode. In particular, a generator 10 having a three-phasemain power winding output 26 that provides power to a balance of thepower system 16, including the power system beyond the generatorterminals, such as transformers, circuit breakers, transmission linesand other electrical loads in the power system. In addition, thegenerator has auxiliary power windings which are represented by theoutputs lines 28 to those windings. The General Electric Company hasdeveloped synchronous generators having both main power windings andauxiliary power windings. In particular, the GENERREX™ excitation systemincludes auxiliary power windings in synchronous generators, such as isdescribed in U.S. Pat. Nos. 4,910,421; 4,682,068 and 4,477,767.

[0010] In addition, auxiliary windings in synchronous generator/motormachines have been proposed in Naval ship propulsion systems. Inparticular, the main generator windings would provide power to electricmotors coupled to the propeller shaft. The auxiliary windings wouldprovide power to the shipboard power distribution system for lights,motors and other ship functions.

[0011] High voltage generators have been developed that operate atnormal transmission line voltages of 40 kV to 400 kV. These generatorsproduce output power in the range of normal transmission line voltage,which is substantially greater than the output voltage range of priorgenerators and well beyond the voltage range of the power supply for astatic start system. High voltage generators have armature windings thatoperate in 40 kV to 400 kV. It is believed to be impractical to couple astatic start drive (which operates in the range of 2.3 kV to 7 kV) tosupply the high voltage windings in a high voltage generator whileproviding reasonable starting torque. Accordingly, there is a need forstatic start drive system which may be coupled to high voltagegenerators.

[0012] In addition, the static start drive system can be applied tosynchronous generators which are being operated as synchronouscondensers. Synchronous condensers are operated in power systems tosupply reactive power to assist in maintaining desired voltage levels.The condensers provide no real power to the system. Synchronouscondensers are generally applied to provide reactive power that leadsreal power to offset or cancel normal lagging reactive power in a powerload system.

BRIEF SUMMARY OF THE INVENTION

[0013] A novel system has been developed which combines a static startdrive with auxiliary power windings to provide a generator with theability to use high voltage generators to start gas turbines, serve as asynchronous condenser, and perform other functions. In one embodiment ofthe invention, a static start drive is connected to the auxiliarywinding of a high voltage synchronous generator. The auxiliary windinghas winding characteristics, e.g., such as voltage and currentcapabilities, that are suitable for connection to a static start drive,or other auxiliary load or excitation system. The main windings of thegenerator may be designed for much higher voltages and would not besuitable for a static start drive.

[0014] Applying a static start drive to an auxiliary power winding of ahigh voltage generator allows the static start system to be used with ahigh voltage generator. Starting a gas turbine using a variablefrequency power supply attached to an auxiliary winding in a highvoltage generator allows the static start technique to be used with awider variety of generators, e.g., high voltage generators than waspreviously available. In addition, providing two windings in thegenerator increases the flexibility in its uses. By separating the mainwindings from the auxiliary windings, the characteristic of each type ofwindings may be selected to best suit different applications of the samegenerator. In particular, the auxiliary windings may be designedspecifically to work in conjunction with the static start drive, and themain windings may be designed to connect to a high voltage transmissionline.

[0015] Alternatively, the auxiliary windings to the static start drivemay have previously existed in the generator for other purposes. Forexample, the GENERREX excitation system employs auxiliary windings thatare separate from the main power windings and are formed by a series ofconductors placed in the stator slots with the main windings. Theseconductors form distinctive three-phase auxiliary windings separate fromthe main windings. By making use of existing auxiliary windings in agenerator, a generator may be converted to include a static start drivesystem without making major internal modifications to the generator. Forexample, a static start system may be added to a high voltage generatorwith existing auxiliary without adding additional stator windings andwithout having to couple the static start drive to the main statorwindings of the generator. In addition, use of auxiliary windings in ahigh voltage generator provides a means suitable for placing balancedcurrents in the winding region of the stator core. The combination ofconductor placement and current forms a rotating magnetic field fordriving the generator as a motor.

[0016] Balanced windings and balanced currents are used to create amagnetic field that rotates within the motor or generator at synchronousspeed. Windings are said to be balanced if they are geometricallyidentical and equally placed around the periphery of the stator core(every 120°). Currents are said to be balanced if the current in each ofthe phases has the same magnitude (amperes), and the time phase of thealternating currents differ by a predetermined angle, typically 120° fora three phase winding. Balanced currents flowing in the balanced windingproduce a magnetic field in the gap between the rotor and stator that isconstant in magnitude and spins in synchronism with the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] These and other features and benefits of the present inventionwill be more fully understood by careful study of the following moredetailed description of preferred exemplary embodiments of theinvention.

[0018]FIG. 1 is a schematic diagram showing a conventional static startdrive system coupled to a generator;

[0019]FIG. 2 is a schematic diagram showing a conventional generationsystem coupled to an auxiliary power winding of a generator;

[0020]FIG. 3 is a schematic diagram of a novel static start systemcoupled to an auxiliary power winding of an auxiliary generator, and

[0021]FIG. 4 is a schematic diagram of a three-phase main windingssystem having taps to provide an auxiliary winding function for agenerator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0022]FIG. 3 shows a schematic diagram that generally shows a generator50 having a stator 51 with both main power windings and auxiliary powerwindings represented by the main power winding armature output 52 andauxiliary power winding armature output 54. In the embodiment shown inFIG. 3, the main power winding armature 52 carries power to a balancedpower load system 56. The load system may be a line transmission voltagesystem typically operating in the range of 40 kV to 400 kV. The voltagesin these ranges may be directly provided from the generator 50 over itsmain power windings 52 to the balanced power system.

[0023] In addition, the generator 50 has an auxiliary power winding 54that provides electrical power via its armature system to a balancedauxiliary power system 58. The auxiliary power system 58 may be a lowvoltage system or other auxiliary power load system. The auxiliary powerload system may constitute the electrical loads in the power plantassociated with the generator. A high voltage generator may output powerat 400 kV, for example, which is too high for use within the plant. Theauxiliary winding provides low voltage power from a high voltagegenerator.

[0024] The generator is able to provide electrical power via its mainpower windings to a power load system 56 which is operated at differentvoltages and currents than is the auxiliary power load system 58 whichis also powered by the generator. The generator drives both main powersystem and auxiliary power system because it has main power windings 52and auxiliary power windings 54.

[0025] A static start drive 60 may be coupled to the auxiliary powerwinding 54 of the generator. The static start drive may be a variablefrequency power supply used to operate the generator as a motor and turnthe rotor at a variable rotational speed proportional to the frequencyof the static start drive. The rotor of the generator turns a driveshaft 61 coupled to a gas turbine 63 to start that turbine. The staticstart drive may also include a loadcommutated inverter (LCI) or pulsewidth modulated (PWM) drive. Components for the variable frequency powerdrive may include rectifiers, diodes and other advanced solid-stateelectronic components. Driving power for the static start drive isprovided from a station auxiliary power bus 62, which may providethree-phase current at 50 Hz or 60 Hz.

[0026] The static start drive is switchably connected to the auxiliarypower winding 54 through a disconnect breaker or switch 64. Similarly,the auxiliary power bus 62 is switchably connected to the balance of theauxiliary power system 58 by a second disconnect breaker or switch 66.The disconnect breaker switch 64 for the static start drive is closed tocouple the static start drive to the auxiliary power windings of thegenerator 50 when the generator is to be used as a motor to start a gasturbine, for example. The disconnect breaker 64 for the static startdrive is closed to engage the static start drive to the auxiliary powerwindings of the generator. At the same time, the disconnect breaker 66between the auxiliary power winding and balanced auxiliary power system58 is opened to disconnect the auxiliary power winding from theauxiliary power system.

[0027] The breakers 64, 66, ensure that while the static start drive 60is providing variable frequency power to the auxiliary power winding ofthe generator, the auxiliary power system 58 is not drawing power fromthe static start drive or generator. In addition, during static startoperations, the auxiliary power system 58 should be disconnected fromthe auxiliary power winding 54 to avoid feeding variable frequency powerinto the auxiliary power system 58. Alternatively, when the static startdrive is disconnected from the auxiliary power winding (such as afterthe gas turbine has been started and is self-sustaining), the firstdisconnect breaker 64 is opened and the second disconnect breaker 66 isclosed. Thus, when the static start is off, the balance of the auxiliarypower system 58 receives current from the auxiliary windings andprovides that current to the auxiliary power bus 62.

[0028] An excitation supply 68 provides magnetizing power to therotating generator field winding 70. Typically, the excitation systemprovides direct current (d.c.) to the field winding 70 at relatively-lowvoltages (300 to 700 volts) compared to the stator voltage in thegenerator 50. During the gas turbine start sequence, the excitationsupply 68 supplies power to the field winding 70 at various levels thatare functions of the turbine-generator speed. The static drive 60 andthe excitation supply 68 are linked with a control circuit to providethe proper power levels during the turbine start sequence. Power for theexcitation supply 68 may be provided from the auxiliary power bus 62.

[0029] The main windings of the generator 50 may have armature voltagesin the range of 40 kV to 400 kV, which is well above the voltagessuitable for static start drive systems. The auxiliary winding may havearmature winding voltages in the range of 2 to 7 kV, which is suitablefor connecting to a static drive system. The winding voltages mayincrease with advances in solid state electronics. Accordingly, thepresent system allows for a static start drive 60 to be coupled to ahigh voltage generator 50. The auxiliary windings 54 may be sized tomatch the variable frequency power supply provided by the static startdrive 60.

[0030]FIG. 4 is a schematic circuit showing an auxiliary power windingformed by tapping the turns of a main power winding. A three-phase mainpower winding system 72 is shown by the three branch windings 74, 75 and76 of the main windings of generator 50. The outputs of the mainwindings 30 78, 80 and 82 provide three-phase power, such as highvoltage power for direct connection to line transmissions. The mainwindings have power taps 84 that connect to the end turns of one of thefirst few turns of the main power winding. These taps allow current tobe extracted from the main windings at a point where the voltage isrelatively low as compared to the voltage across all of the turns in themain windings. By tapping the first few turns of the main power winding,an effective auxiliary winding may be created which provides polyphase(e.g., three phase), low-voltage and low-amperage current at anauxiliary connection bar 86. The auxiliary connection bar 86 may be usedto couple to the first breaker or switch 64 and to the balance of theauxiliary power system 58.

[0031] The present static drive system coupled to auxiliary windings maybe generally applied to polyphase synchronous electrical machines duringtheir start-up phases. As described above, the present system isparticularly suitable for use with generators that operate in connectionwith gas turbines and provide output voltages that are substantiallygreater than those voltages of typical power conversion systems, such asrectifiers, LCI and PWM drives. Moreover, the present system may beemployed with a variety of auxiliary windings, including those that tapmain power windings, are formed of additional conductors included withmain windings in a stator, and other polyphase synchronous electricalmachines that have auxiliary winding systems which match static startdrive systems.

[0032] While particular exemplary embodiments of the present inventionhave been described and illustrated, it should be understood that theinvention is not limited to the disclosed exemplary embodiments.Modifications and variations may be made by persons skilled in the artwhile still retaining some or all of the advantages of this invention.The present invention is intended to include any and all suchmodifications within the spirit and scope of the following claims.

What is claimed is:
 1. A electric power generator system comprising agenerator having a stator including a main power winding and anauxiliary power winding; said main power winding connectable to a firstpower system; said auxiliary power winding connectable to an auxiliarypower system, and a variable frequency power source connectable to theauxiliary power winding.
 2. An electric power generator system as inclaim 1 wherein the generator further includes a drive shaft that isconnectable to a gas turbine.
 3. An electric power generator system asin claim 1 further comprising an excitation supply providing power to afield winding in said generator, and said excitation supply having anoutput connected to the variable frequency power source.
 4. An electricpower generator system as in claim 1 further comprising a breaker orswitch coupling the variable frequency power supply to the auxiliarywinding.
 5. An electric power generator system as in claim 1 wherein thegenerator is a three-phase generator with three main armature windingshaving a rated output voltage in a range of 40 kilovolts (kV) to 400 kV,and three auxiliary windings having a rated output voltage of less than10 kV.
 6. An electric power generator system as in claim 1 wherein theauxiliary winding comprises a portion of the main winding.
 7. Anelectric power generator system as in claim 1 wherein the auxiliarywinding comprises several turns of the main winding and a tap providingan external connection to the several turns.
 8. A method for starting agas turbine using a polyphase electric power generator comprising thesteps of: a. connecting a variable frequency power supply to auxiliarywindings of the generator; b. applying current to the auxiliary windingsof the generator to turn a drive shaft of the generator; c. rotating agas turbine with the turning rotor of the generator to start the gasturbine, and d. once started, the gas turbine turns the generator tocause the generator to produce electric power from the main armaturewindings.
 9. A method for starting a gas turbine using a polyphaseelectric power generator as in claim 8 further comprising the step ofdisconnecting the variable frequency power supply from the auxiliarywindings after the gas turbine is started.
 10. A method for starting agas turbine using a polyphase electric power generator as in claim 8further comprising the step of increasing the frequency of the powerapplied by the variable frequency power supply to accelerate therotational speed of the drive shaft as the gas turbine is being started.11. A method for starting a gas turbine using a polyphase electric powergenerator as in claim 8 wherein the generator is a three-phase generatorhaving three main windings having an output voltage in a range of 40kilovolts (kV) t o 400 kV, and three auxiliary windings having an outputvoltage of less than 10 kV.
 12. A method for starting a gas turbineusing a polyphase electric power generator as in claim 8 furthercomprising the step of disconnecting the variable frequency power supplyafter the gas turbine is started and connecting the auxiliary windingsto an auxiliary power system.